Added get_wsi_at_mpp tests; fixed a few bugs

Author: NikolasSchmitz

monai/data/wsi_reader.py

@@ -1459,4 +1459,4 @@ def _resize_to_mpp_res(self, wsi, closest_lvl, mpp_list, user_mpp: tuple):
         closest_lvl_wsi = pil_image.fromarray(wsi.pages[closest_lvl].asarray())
         closest_lvl_wsi = closest_lvl_wsi.resize((target_res_x, target_res_y), pil_image.BILINEAR)
 
-        return closest_lvl_wsi
+        return closest_lvl_wsi